‘Spiders’ keep an eye on water behaviour

Data-logging and telemetry units are helping Sydney Catchment Authority keep a close eye on NSW’s most important catchments.

Sydney Catchment Authority (SCA) manages and protects the city’s drinking water catchments and associated infrastructure and supplies bulk water to its customers, which include Sydney Water and a number of local councils. With more than four million people relying on SCA for their water, it is essential that the authority keeps a close watch on the behaviour of the catchment and the quality of the water.

ALS Environmental, through its field services office, provides specialist services to SCA to monitor and analyse many aspects of the catchment. Occasionally, when faced with a challenge, ALS’s Monitoring and Technical Services Group needs to devise new methods to take measurements or find a way to send data from remote locations.

One such challenge arose when SCA wanted to measure water level and quality at a number of points along the Warragamba River where there was no GSM or NextG coverage.

While there had been a satellite system used there in the past, it was very expensive, provided data only once a day and was somewhat unreliable. ALS Environmental approached Halytech for a solution and was told that, with a little work, the Spider Satellite data logger would be suitable.

“The Spider is an extremely versatile device and we have customised it to suit the requirements of many of our customers,” said Steven Palos of Halytech. “Working with ALS was easy as they knew what they wanted and trusted us to come up with a solution.”

A significant part of the challenge was that the river was in a deep ravine with a relatively small view of the sky. For a geostationary satellite system to be useful, it needs to be visible from the site from where data is to be collected. In a steep-sided ravine such as Warragamba River, it is sometimes difficult to get access to a suitable geostationary satellite.

An orbiting satellite on a network such as Iridium is constantly moving and completes its orbit in about 100 minutes. In order to be able to communicate reliably, a ground station must be able to ‘see’ the satellite for enough time to transmit its data. At some locations on Warragamba River, the window of opportunity is very small, increasing the possibility that data transmission will be interrupted when the satellite passes from view.

Halytech developed a data compression technology which reduced the data to be transmitted to very small packets which, once communications with a passing satellite has been established, can be delivered in a few seconds. This makes it possible to get good quality data from remote sites having poor sky access with great reliability. The chosen site for the initial trial was regarded by ALS and SCA as the most difficult site in the catchment.

According to Anthony Skinner, ALS Environmental’s Sydney manager, “The Spider Satellite is the primary data logger and telemetry device on all priority one sites not covered by NextG.” Referring to the first site, he went on to say, “We haven’t missed a single data point from that site in six years.” Impressed with the performance of the Spider satellite, ALS installed six more at other sites along the river.

Confident of the abilities of the satellite to deliver data from the remotest of sites, ALS again employed the product for the monitoring of thermistor chains used to measure water temperature in Warragamba Dam. One of the challenges faced in implementing this solution was the need to gather data from more than 60 thermistors. At the time, the satellite could handle only eight channels but Halytech was able to increase this to 65.

Mounted on rafts, the satellite read data from the thermistor chains using the SDI-12 protocol and sent it to ALS over the Iridium network. Arriving every hour into a Hydstra system, the data was checked for quality and uploaded to a web portal for use by SCA as required.